The present invention relates to gypsum-based composite articles which are usable as fireproof architectural materials, and methods for producing such articles.
Hitherto, cement (portland cement) board or gypsum board made of hard gypsum has generally been used as a fireproof and earthquake-proof architectural material. Gypsum board is advantageously less than portland cement board in weight, if both boards have the same dimensions. Furthermore, gypsum board becomes advantageously more flexible, as it becomes thinner in thickness. A mineral of gypsum contains a main component of calcium sulfate and is smell-less, colorless and atoxic. Gypsum products are chiefly used in the building industry and as a retarder in portland cement. Limited amounts of gypsum products are used as the mold material for producing ceramic products and in the field of medicine, particularly dentistry. In general, gypsum is obtained by purifying the natural gypsum mineral. In recent years, however, gypsum has mainly been obtained as a by-product in a process for producing phosphoric acid by decomposing rock phosphate with sulfuric acid. This gypsum is called chemical gypsum, particularly phosphogypsum.
There are two types of gypsum, that is, dihydrate gypsum (CaSO.sub.4.2H.sub.2 O) and anhydrous gypsum (CaSO.sub.4), and most of the natural and chemical gypsum belongs to dihydrate gypsum. In industrial use of gypsum, dihydrate gypsum is ground and then calcined at high temperatures to remove part of the water of crystallization. The obtained calcined product, hemihydrate gypsum (CaSO.sub.4.1/2H.sub.2 O), is generally known as plaster of paris. When the powdered hemihydrate gypsum is mixed with water to form a paste or slurry, the calcining reaction is reversed. With this, the paste or slurry sets and hardens to a hard, coherent, stable solid made of dihydrate gypsum, with expansion and heat generation. According to Japanese Industrial Standard (JIS), the above-mentioned calcined gypsum (plaster of paris) is classified into four grades, that is, Special high grade, A-grade, B-grade and C-grade. The amount of water to be mixed with calcined gypsum must be specifically changed, depending on the grade of calcined gypsum. In this regard, a calcined gypsum with higher grade requires less amount of water in the preparation of the slurry, and the set plaster prepared from such calcined gypsum having higher grade becomes superior in strength and durability. The set plaster (gypsum product) varies in strength, density, expansion coefficient and porosity, depending on the type of additives and the mixing time for mixing calcined gypsum with water, as well as the amount of water to be mixed therewith. Thus, it is required in the preparation of gypsum products to carefully select the optimum materials and the optimum conditions.
Hitherto, there have been known reinforced gypsum products each having therein a reinforcing material of paper or Manila hemp. This reinforcing material is embedded in the gypsum product and serves as a binding or joining material to improve the strength of the gypsum product. These reinforced gypsum products, however, are generally insufficient in fireproofness and in resistance to external force (e.g. earthquake) acting thereon and to severe environmental conditions (e.g. abrupt change of the ambient temperature).
Architectural wood or plywood boards are generally used as floor material, wall material and trim members of building interior. These boards, however, are not sufficient in sound insulation and fireproofness.